1. Field of Invention
This invention relates generally to circuits for driving inductive loads such as solenoids and, more particularly, to circuits wherein power consumption is minimized by operating a power stage in a switching mode.
2. Description of the Prior Art
The above-identified Reddy cases disclose injector drive circuits wherein the current is controlled to first regulate the voltage across the injector coil until after the current has built up enough to open or "pull in" the injector armature and to thereafter regulate the current at a holding level in excess of a closing or "drop out" current but considerably less than the "pull in" current. These drive circuits render the injector opening and closing times, and therefore the fuel supplied therebetween, substantially independent of variations in the power supply and variations in the unit-to-unit voltage drops across the power stage.
However, in regulating first the voltage and then the hold current, the power stage experiences a voltage drop thereacross corresponding to the difference between the voltage of the power supply and the voltage at the injectors. The power stage therefore consumes and must dissipate power at a level increasing with the number of injectors being driven at one time and the current required to operate each injector. The heat dissipation and therefore the heat sink and temperature ranges associated with these power stages also increase accordingly to the point that the heat sink is often larger than all of the other elements of the electronic control unit leading up to the injectors. Moreover, the temperature cycles produced by the dissipation decrease the life while increasing the mounting costs of the semi-conductor elements comprising the power stage. It is therefore desirable to reduce the power dissipated by the power stage.
The United States patent to Paine U.S. Pat. No. 3,549,955 discloses a circuit for minimizing power consumption by operating the power stage fully ON so that there is a very small voltage drop thereacross until a pull-in current level is detected and thereafter operating the power stage in a switching mode where it is either fully ON or OFF to maintain a hold-in current level in excess of the drop out level. More specifically, the current is alternately increased to an upper hold-in level in excess of the drop out level and is then allowed to decay slowly through the solenoid coil to a lower hold-in level still in excess of the drop-out level. Thereafter, the power stage is switched fully ON again until the upper hold-in level is again detected.
The United States patent to Ule U.S. Pat. No. 3,896,346 discloses an inductive load driver circuit of the type disclosed in the Paine patent but wherein power consumption is further minimized by returning the energy in the collapsing solenoid field to energy storage means in the form of the power supply or a second solenoid.
Citing the Paine patent, the United States patent to Stewart U.S. Pat. No. 4,041,546 discloses a solenoid drive circuit including capacitor timing means for disconnecting the driving voltage for fixed intervals between each application of hold current.
To minimize power consumption, switching-mode decay rates effected by circuits of the type disclosed in Paine, Ule, and Stewart patents must be sufficiently slow to maintain the current above the drop-out level while the power stage is OFF. Using such circuits to control a fuel injector would render the turn off time of the injector, and therefore the fuel delivered thereby, subject to when in the decay cycle it is desired to close the injector. For example, if the end of the injector actuation command coincided with the instant that the current exceeded the upper hold-in level, then the closing time would be the time required to decay to the lower hold-in level plus the time required to decay therefrom to the drop-out level. If the end of the actuation command coincided with the instant that the current fell below the lower hold in level, then the closing time would be just the time required for the current to decay to the drop out level.
Use of circuits of the type disclosed in the Paine, Ule, and Stewart patents to control a fuel injection valve would therefore incur not only the variations in oepning times eliminated by the circuits of the above-identified Reddy cases but would also introduce variations in closing times that would offset a substantial portion of all the variations eliminated by the Reddy circuits.
It is therefore desirable to provide an improved circuit wherein the solenoid current is held above a drop-out level by switching the power stage between fully ON and OFF conditions to reduce power consumption and then rendering the closing time independent of decay rates associated with such hold level switching.